Electrographic printing is a well known printing process in which an electrostatic latent image is made to attract electrostatic marking particles in a toner. The toner can be of the dry type or of the liquid type. Electrographic printing is particularly applicable when only a relatively small number of prints are required, or when the subject matter is frequently changed, or when part of the subject matter needs to be sequentially changed.
Dry powder toners have many disadvantages when used in such a process. For high speed, long run printing, cost per page is a principle consideration. In particular, the cost of fusing the image to paper or any other desired substrate significantly contributes to the running costs of such a printer. Other objections are related to the problem of dusting; dust, or fine or small particles of toner are prone to escape from the developer, and these deposit onto any surface both within and outside the printing device, causing mechanical failures within the device and environmental problems outside the device. This problem becomes severe when such printing devices are run at high speed. Other disadvantages include, cost of the general maintenance of the press and cost of the dry powder toner.
It is known that latent electrostatic images can be developed with toner particles dispersed in insulating or non-polar liquids. Such toner particles normally comprise colouring matter such as pigments which have been ground with or otherwise combined with dispersing resins or varnishes or the like. Additionally polarity control or charge directing agents are usually included to control the polarity and charge mass ratio of the toner particles. Said dispersed materials are known as liquid toners or liquid developers. In use, a liquid developer is applied to the surface of a support bearing a charge pattern to develop an electrostatic image on the support.
The developed latent image is fixed to the surface, or upon transfer to a subsequent substrate, generally by heating to temperatures above room temperature to fuse the toner image to the substrate. A number of methods can be employed to achieve fixation to a substrate, for example, IR or UV radiation, solvent vapour, or a number of other techniques or combinations thereof commonly employed by those skilled in the art. As can be appreciated, the above mentioned methods of fixing the toner image to the substrate can be characterised by high energy consumption and or can be environmentally undesirable, slow, or a combination of these.
Liquid developer toner particles can also have combined therewith fixing materials which are commonly but not necessarily thermoplastic polymeric materials. Such fixing materials when incorporated into the toner may affect other physical properties, thereby reducing the mobility of said toner particles thus making such a toner not suitable for high speed electrographic printing.
Devices using liquid electrographic printing also have a number of objectionable problems, especially when these devices are required to operate at high speed. The main problem is in regard to the solvent carry-out. The term solvent carry-out relates to the quantity of solvent or carrier which is trapped within the paper. Such solvent subsequently evaporates during image fusing, giving rise to atmospheric pollution and also adding significantly to production costs. A further disadvantage of liquid toning is the tendency for deposition of colouring matter in non-image or background areas which results in a general discolouration of the copy, normally referred to as background fog.
To overcome the above mentioned undesirable characteristics of liquid developers, a number of methods have been proposed.
According to U.S. Pat. No. 4,659,640 issued Apr. 21, 1987, to Santilli, an electrostatic developer liquid is disclosed wherein a wax dispersed in the liquid carrier is described, thus rendering the developer self-fixing at room temperature.
Also, disclosed in U.S. Pat. No. 4,507,377 issued Mar. 26, 1985, to Alexandrovich, a developer composed of a compatible blend of at least one polyester resin and at least one polyester plasticiser. This composition providing a self-fixing liquid electrographic developer.
The above cited examples, although providing self fixing, impose process limitations. The formulations disclosed and self-fixing adjunct therein, limit the electrophoretic development rate of the liquid toner particles. In a high speed electrographic printing press such formulations would not be able to sustain adequate density and self-fixing characteristics at elevated printing speeds in excess of 1 meters per second.
The teachings of U.S. Pat. No. 5,558,970 issued Sep. 24, 1996 to Landa et al., discloses a method of treating a developed image prior to transfer to a final substrate. This process involves enhancing the cohesiveness of the developed image by either physical or chemical means. The application of heat from a radiant energy source for example, or by catalysis are two methods disclosed. It can be appreciated however by those skilled in the art that the above disclosed teachings have the drawback of further expenditure of energy as well as increased solvent evaporation giving rise to greater atmospheric pollution and also adding to production costs. The addition of a catalyst to the developed image, for example, adds a process step which complicates and further limits the process speed of the printer. The image on the final substrate still requires further fusing in addition to the above disclosed step of increasing the cohesiveness of the developed image, again further limiting the process speed of a printer.
U.S. Pat. No. 3,907,423 issued Sep. 23, 1975, to Hayashi et al., teaches a method and means of extracting excess developer liquid to reduce solvent carry-out by mean of an extractor roller.
Further, U.S. Pat. No. 4,286,039 issued Aug. 25, 1981, to Landa et al., teaches of a method of using a deformable polyurethane roller as a squeegee or blotting roller.
U.S. Pat. No. 5,023,665 issued Jun. 11, 1991, to Gundlach, discloses an apparatus for removal of excess carrier liquid, consisting of an electrically biased electrode having a slit therein coupled to a vacuum pump to remove unwanted carrier liquid.
Again, the above cited disclosures, although providing reduced solvent carry-out, also impose process speed limitations. The methods disclosed therein, are not sufficiently reliable at high machine process speeds. In a high speed electrographic printing press such methods would not be able to sustain adequate solvent extraction at elevated printing speeds in excess of 1 meters per second.
U.S. Pat. No. 3,722,994 issued Mar. 27, 1973, to Tanaka et al., teaches of a method of using a corona discharge device for removing any residual developing solution from an imaging member.
Yet again, the above cited disclosure, although providing reduced solvent carry-out, has a number of adverse affects, which may be time dependent, on many latent image bearing members as would be appreciated by those skilled in the art. The method disclosed is also not sufficiently reliable at high machine process speeds. In a high speed electrographic printing press such a method would not be able to sustain adequate solvent extraction at elevated printing speeds in excess of 1 ms-1. In a yet further contrapositive, this method produces substantial environmentally undesirable ozone generation.